Traditional Ziegler-Natta type catalysts for the polymerization of olefins have been known since the 1950's. Generally, these catalysts comprise a transition metal halide compound, particularly one of titanium and chloride, and a metal alkyl cocatalyst, particularly an aluminum alkyl cocatalyst. The traditional catalyst systems are generally comprised of several chemically distinct active metal sites which produce different polymeric materials (molecular weight comonomer, etc.) under steady state reactor conditions. During the last 30 years of development, traditional Ziegler-Natta catalysts have been optimized and provide low cost routes to a variety of commercially important polyolefins, however further improvements over the control of key polymer structure parameters such as molecular weight distribution (MWD), composition distribution (CD), end group functionality, sequence distribution and polar comonomer compatibility are still needed.
Recent progress to improve the Ziegler-Natta system has been directed towards the production of soluble, single sited olefin polymerization catalysts derived from transition metal precursors where the halide ligands used in tradition catalysts have been replaced by bulky, organic ancillary ligand systems, such as cyclopentadienyl (Cp) derivatives. In contrast to simple halide ligands, the bulky ancillary, ligand systems are not susceptable to removal or change during polymerization, stabilize a single form of the catalyst, and can be modified in a rational way to change the properties of the catalyst center. The development of high activity catalyst systems derived from bis- and mono- Cp stabilized Group 4 metal precursors and alumoxanes is now well documented. Despite the fact that the cocatalyst (and therefore the resulting catalyst) is a chemically complex mixture of aluminum alkyl reagents, the active catalyst generally behaves as a single sited catalyst and produces narrow MWD polyolefins.
Recent advances in the an have led to the development of well defined (discrete), ancillary ligand stabilized single-sited olefin polymerization catalysts of the Group 3 and 4 transition metals. These systems do not rely on the use of alkyl aluminum cocatalysts, and offer improvements in terms of cost and product versatility. Examples of mono-Cp and bis Cp based Group 3 and 4 discrete catalysts are represented below. ##STR1##
The neutral Group 3 systems were prepared and studied by Ballard et al. (J. Chem. Sec., Chem. Comm., (1978) p. 994-995) and Bercaw et al. (Organometallics, Vol. 9 (1990) pp. 867-869). The work of Bercaw and others in this field has demonstrated that the neutral, three-coordinate systems tend to dimerize to form inactive four-coordinate systems when the size of the ancillary ligands is decreased. The use of bulky Cp ligands to sterically prevent self-dimerization and maintain catalytic activity has been successful, but the resulting systems are often too sterically hindered to allow for the incorporation of larger olefin monomers.
The discovery of isostructural and isoelectronic discrete mono- and bis-Cp based Group 4 cationic systems, are discussed in European Patent Application (EPA) 277,003 and EPA 277,004, PCT International Application WO 92/00333 and EPA 418,044. These cationic systems are stabilized by the ancillary ligand system and are isolated as the salts of specially designed compatible non-coordinating anions. The development of compatible non-coordinating anions, as disclosed in EPA 277,003 and 277,004 represents a major advance in the field because they provide a means by which highly reactive coordinatively unsaturated organometallic cations can be generated and isolated. Although the ionic Group 4 catalysts are greatly improved over the organoaluminum activated systems in terms of producing polymers having greater molecular weight, enhanced molecular weight distribution and composition distribution control, there exists a need to provide a catalyst with improved thermal stability, product versatility, and tolerance to polar functionality.
The need for improved single sited olefin polymerization catalysts is evident, particularly in terms of chemical stability (e.g., air stability) and tolerance to polar functionality. Improvements in these properties would yield catalysts which could tolerate greater levels of impurities in the monomer feeds, and which could incorporate polar comonomers into a polyolefin backbone. For this reason, it would be desirable to develop olefin polymerization catalyst comprised of later transition metals. The best studied and well defined late metal catalysts for the polymerization of ethylene has been developed by Brookhart and coworkers (J. Am. Chem. Soc. 1985, 107, 1443-1444). These systems, Cp*Co(L)R.sup.+ BX.sub.4.sup.-, are comprised of cationic cobalt (III) complexes containing a ligand, Cp*, a neutral datively bound ligand, L, and a reactive sigma-bound alkyl with the charge on the metal center balanced by a borate anion BX.sub.4-. While these catalyst systems offer some potential advantages, particularly with respect to compatibility with polar functionalities and for the production of very narrow molecular weight distributions, they suffer from very low activity and yield only a single polymer chain per metal atom. A goal is to develop polymerization catalysts which combine the activity and yield of cationic Group 4 systems with the selectivity and functional group tolerance of later metal systems.